High velocity actuators have been employed in a wide number of areas. For example, high rate (velocity) actuators have been used to perform tensile rate dependency tests on metals and plastics, and for performing compression tests to determine properties related to forging of materials. High velocity actuators have been made to impact the muzzle of an artillery weapon to test the recoil mechanism without actually firing a projectile. In other uses, a sled or table has been accelerated to a high velocity. The sled or table with an installed test article impacts a spring, damper, or mass to subject the test article to a controlled shock test. Actuators have also been used to accelerate simulated heads to a desired velocity to test the properties of helmets, padded dashboards, and other objects designed to protect people in crashes. Actuators have been used to accelerate a mass to a controlled velocity and allowed it to impact a hydraulic cylinder to produce extremely high pressures in an artillery gun breech chamber to perform a fatigue test on the breech without firing the gun. Controlled actuators have been used to produce accelerations duplicating the acceleration of the passenger compartment of a car in a crash.
Such applications, described above, have involved the use of only a single actuator. Various methods were used for controlling the actuators, and included the use of a face seal, with the actuator piston acting as a valve. After a triggered release, the actuator piston allowed free flow of fluid from an accumulator into the actuator. Another method was the use of a high flow servo valve to control the flow from an accumulator to a small area actuator to provide high velocities. Fast opening solenoid valves have also been used, to provide uncontrolled flow from an accumulator into an actuator. Also employed have been servo controlled poppet valves. These are similar to the servo valve, but exhibit higher flow capability.
Another system, used to simulate the effect of a terrorist bomb on structural components of civil structures, employs multiple actuators to accelerate masses to a velocity for simultaneous impact on a structural element such as a reinforced concrete column. The impact velocity and mass of the impactors transfers an impulse (momentum) to the structure to duplicate the impulse measured from actual explosions. Control of the actuators was by servo-controlled poppet valves. This allowed starting all actuators simultaneously and provided the ability to adjust the command to the individual valves to achieve desired velocities and near simultaneous impact on all actuators.
Impact momentum is mass times velocity. The impulse and energy transferred to a specimen is a function of the ratio of the impact mass to the specimen mass and the losses in the impact spring. The best efficiency of energy transfer to the specimen during the impulse occurs when the two masses are close to equal.
In order to evaluate higher strength terrorist targets where the explosive might be set off very close to the structure, it is necessary to provide higher impact velocity. Doubling of the velocity is required to achieve four times the energy. However, previous blast generators have been limited to a velocity of about 30 meters/second. To increase the velocity to double, increasing the actuator stroke length is not an option due to piston rod weight and piston rod buckling considerations. To provide double the velocity in the same acceleration distance requires doubling the acceleration. The piston area must be doubled, requiring four times the flow at maximum velocity. Increasing the hydraulic pressure is possible, but hydraulic valves and fittings are not practical, since they are very expensive for pressures beyond normal working pressures for commercial hydraulic equipment.